Magnetic recording medium

ABSTRACT

This invention relates to an improved magnetic recording medium having an increased durability as well as a decreased head abrasion, which comprises a magnetic recording layer on the surface of a non-magnetic support, the magnetic recording layer containing a ferromagnetic powder, an abrasive having a Mohs&#39; hardness of at least 6, a non-magnetic powder which is granular α-Fe 2  O 3  and a fatty acid and fatty acid ester.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved magnetic recording medium and moreparticularly, it is concerned with a magnetic recording medium having animproved coating durability as well as a decreased head abrasion.

2. Description of the Prior Art

In order to improve the abrasion resistance of a coating film in amagnetic recording medium, it has hitherto been conducted to addabrasive particles having a large Mohs' hardness to the magnetic layer,but this method has the drawback that the abrasion of a magnetic head isincreased, although the durability of the coating is improved.Furthermore, in order to improve the abrasion resistance of a magneticlayer and the abrasion suppressing action of a magnetic head, there hasbeen proposed a magnetic recording medium having a non-magnetic powderwith a Mohs' hardness of at least 9 and granular α-Fe₂ O₃ (JapanesePatent Application OPI No. 129935/1980), which still shows, however, aninsufficient durability of magnetic layer.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnetic recordingmedium whereby the above described drawbacks of the prior art can beovercome.

It is another object of the present invention to provide a magneticrecording medium having an improved durability of the coating film, inparticular, even at a low temperature.

It is a further object of the present invention to provide a magneticrecording medium capable of giving a decreased head abrasion and a longstill life even after repeated use.

These objects can be attained by a magnetic recording medium comprisinga magnetic recording layer on the surface of a non-magnetic support, inwhich the magnetic recording layer contains (i) a ferromagnetic powder,(ii) an abrasive having a Mohs' hardness of 6 or more, (iii) anon-magnetic powder, granular α-Fe₂ O₃ in a proportion of 5 to 30% byweight to the ferromagnetic powder and (iv) a fatty acid and fatty acidester.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provide a magneticrecording medium comprising a magnetic recording layer on the surface ofa non-magnetic support member, characterized in that the magneticrecording layer contains (i) at least one of ferromagnetic powders, (ii)at least one of abrasives with a Mohs' hardness of at least 6, (iii)granular α-Fe₂ O₃ in a proportion of 5 to 30% by weight to theferromagnetic powder and (iv) at least one each of fatty acids and fattyacid esters.

Useful examples of the ferromagnetic powder with can be used in thepresent invention are, for example, γ-Fe₂ O₃, Co-doped γ-Fe₂ O₃, Fe₃ O₄,Co-doped Fe₃ O₄, Berthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄ (FeO_(x) :1.33<x<1.50), Co-doped Berthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄(FeO_(x) : 1.33<x<1.50), CrO₂, Co-Ni-P alloys, Co-Ni-Fe alloys,Co-Ni-Fe-B alloys, Fe-Ni-Zn alloys, Fe-Mn-Zn alloys, Fe-Co-Ni-P alloysand Ni-Co alloys, as described in Japanese Patent Publication Nos.14090/1969, 18372/1970, 22062/1972, 22513/1972, 28466/1971, 38755/1971,4286/1972, 12422/1972, 17284/1972, 18509/1972, 18573/1972, 10307/1964and 39639/1973; U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194,3,242,005, 3,389,014; British Pat. Nos. 752,659 and 1,007,323; FrenchPat. No. 1,107,654 and West German OLS No. 1,281,334.

The ferromagnetic powder has a particle size of about 0.2 to 1 micron inlength with a length to width ratio of about 1/1 to 20/1.

Typical abrasive agents which can be used in the present inventioninclude fused alumina, silicon carbide, chromium oxide, corundum,synthetic corundum, diamond, synthetic diamond, garnet, emery(predominant component: corundum and magnetite) and the like, which havea Mohs' hardness of 6 or more, preferably 8 or more. These abrasiveagents have generally a mean particle size of 0.05 to 5 microns,preferably 0.1 to 2 microns and are generally added in a proportion of 1to 10 parts by weight to 100 parts by weight of the magnetic powder.Examples of the abrasives are described in Japanese Patent ApplicationNo. 26749/1973, U.S. Pat. Nos. 3,007,807, 3,041,196, 3,293,066,3,630,910 and 3,687,725; British Pat. No. 1,145,349; and West GermanPat. Nos. 853,211 and 1,001,000.

Non-magnetic powder, granular γ-Fe₂ O₃, is added in a proportion of 5 to30% by weight, preferably 10 to 25% by weight to the magnetic powder.Its grain size is preferably 0.2 to 1 micron.

As the fatty acid, there is generally used at least one of saturated orunsaturated fatty acids having 6 to 22 carbon atoms, preferably 12 to 18carbon atoms in a proportion of 0.1 to 2% by weight to the magneticpowder. Examples of the fatty acid are capric acid, lauric acid,myristic acid, oleic acid, stearic acid, erucic acid and behenic acid.

As the fatty acid ester, there is generally used at least one of fattyacid esters having a melting point of 60° C. or lower, preferably 40° C.or lower in a proportion of 0.1 to 2% by weight to the magnetic powder.Examples of the fatty acid ester are methyl stearate, ethyl stearate,butyl stearate, amyl stearate, butyl palmitate, butyl myristate, oleyloleate and butyl laurate.

The magnetic recording medium of the present invention is capable ofgiving the following advantages or merits by the above described fourparticular components in combination:

(1) The durability of a coating film at a low temperature is excellent.

(2) The head abrasion at a low temperature is decreased.

(3) Damages such as breakage and scratch of a head are suppressed.

(4) Since a lubricant is constantly fed from the magnetic layer to ahead, touch of the tape to the head is held well, thus making smooth thetape running property in a VTR running system and giving a good imagequality.

As will be apparent from Comparative Examples, the above describedmerits cannot be given if a part of the essential components of thepresent invention is not used or if other dispersing agents orlubricants are used instead of the fourth component of the presentinvention.

Production of the magnetic recording medium according to the presentinvention is carried out in conventional manner by blending theforegoing four components with a binder, coating solvent and otheradditives such as abrasives, dispersing agents, lubricants, antistaticagents, etc., kneading, coating onto a non-magnetic support and thensubjecting to a surface finishing treatment.

Suitable binders which can be used in the present invention includehitherto known thermoplastic resins, thermosetting resins and mixturesthereof.

Suitable thermoplastic resins are those which have a softening point ofabout 150° C. or lower, a mean molecular weight of about 10,000 to200,000 and a degree of polymerization of about 200 to 2,000, forexample, vinyl chloride-vinyl acetate copolymers, vinylchloride-vinylidene chloride copolymers, vinyl chloride-acrylonitrilecopolymers, acrylate-acrylonitrile copolymers, acrylate-vinylidenechloride copolymers, acrylate-styrene copolymers,methacrylate-acrylonitrile copolymers, methacrylate-vinylidene chloridecopolymers, methacrylate-styrene copolymers, urethane elastomers,polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymers,butadiene-acrylonitrile copolymers, polyamide resins, polyvinyl butyral,cellulose derivatives such as cellulose acetate butyrate, cellulosediacetate, cellulose triacetate, cellulose propionate, cellulose nitrateand the like, styrene-butadiene copolymers, polyester resins,chlorovinyl ether-acrylate copolymers, amino resins, various syntheticrubber based thermoplastic resins and mixtures thereof. Examples ofthese resins are described in Japanese Patent Publication Nos.6877/1962, 12528/1964, 19282/1964, 5349/1965, 20907/1965, 9463/1966,14059/1966, 16985/1966, 6428/1967, 11621/1967, 4623/1968, 15206/1968,2889/1969, 17947/1969, 18232/1969, 14020/1970, 14500/1970, 18573/1972,22063/1972, 22064/1972, 22068/1972, 22069/1972, 22070/1972 and27886/1973, U.S. Pat. Nos. 3,144,352, 3,419,420, 3,499,789, 3,713,887,etc.

Suitable thermosetting resins have a molecular weight of about 200,000or less as a coating solution and when heated after coating and drying,the molecular weight becomes infinity due to reactions such ascondensation, addition and the like. Of these resins, preferred resinsare those which do not soften or melt before the resin thermallydecomposes. Representative examples of these resins are phenol resins,epoxy resins, polyurethane hardening type resins, urea resins, melamineresins, alkyd resins, silicone resins, acryl based reactive resins,mixtures of high molecular weight polyester resins and isocyanateprepolymers, mixtures of methacrylic acid salt copolymers anddiisocyanate prepolymers, mixtures of polyester polyols andpolyisocyanates, urea-formaldehyde resins, mixtures of low molecularweight glycols, high molecular weight diols and triphenylmethanetriisocyanates, polyamine resins and mixtures thereof. Examples of theseresins are described in, for example, Japanese Patent Publication Nos.8103/1964, 9779/1965, 7192/1966, 8016/1966, 14275/1966, 18179/1967,12081/1968. 28023/1969, 14501/1970, 24902/1970, 13103/1971, 22065/1972,22066/1972, 22067/1972, 22072/1972, 22073/1972, 28045/1972, 28048/1972,28922/1972, U.S. Pat. Nos. 3,144,353, 3,320,090, 3,437,510, 3,597,273,3,781,210, 3,781,211, etc.

These binders can be used individually or in combination with each otherand other additives can be added to the binders. The mixing ratio byweight of a ferromagnetic powder and a binder is 10 to 400 parts byweight, preferably 30 to 200 parts by weight of the binder to 100 partsby weight of the ferromagnetic powder.

Suitable dispersing agents are metallic soaps comprising the alkalimetal (Li, Na, K, etc.) salts or the alkaline earth metal (Mg, Ca, Ba,etc.) salts of fatty acids containing about 12 to 18 carbon atomsrepresented by the general formula R₁ COOH wherein R₁ is an alkyl groupcontaining about 11 to 17 carbon atoms; and lecithin, etc. In addition,higher alcohols containing 12 or more carbon atoms and the sulfatesthereof can be used. These dispersing agents are generally used in aproportion of about 1 to 20 parts by weight per 100 parts by weight of abinder. These dispersing agents are described in Japanese PatentPublication Nos. 28369/1964, 17945/1969 and 15001/1973 and U.S. Pat.Nos. 3,387,993 and 3,470,021.

Suitable lubricants which can be used in the present invention includesilicone oils, carbon black, graphite, carbon black graft polymers,molybdenum disulfide and tungsten disulfide.

Antistatic agents which can be used in the present invention includeelectrically conductive powders such as graphite, carbon black andcarbon black graft polymers; natural surface active agents such assaponin; nonionic surface active agents such as alkylene oxide based,glycerol based and glycidol based surface active agents; cationicsurface active agents such as higher alkylamines, quaternary ammoniumsalts, heterocyclic compounds, e.g. pyridine and the like; phosphoniums,sulfoniums and the like; anionic surface active agents containing acidgroups such as carboxylic acid groups, sulfonic acid groups, phosphoricacid groups, sulfate groups, phosphate groups and the like; andamphoteric surface active agents such as sulfates or phosphates of aminoacids, aminosulfonic acids, amino alcohols and the like; etc.

Examples of the surface active agents which can be used as antistaticagents are described in U.S. Pat. Nos. 2,271,623, 2,240,472, 2,288,226,2,676,122, 2,676,924, 2,676,975, 2,691,566, 2,727,860, 2,730,498,2,742,379, 2,739,891, 3,068,101, 3,158,484, 3,201,253, 3,210,191,3,294,540, 3,415,649, 3,441,413, 3,442,654, 3,475,174 and 3,545,974,West German Patent Application (OLS) No. 1,942,665, British Pat. Nos.1,077,317 and 1,198,450, Ryohei Oda et al., "Kaimen Kassei Zai no Goseito sono Oyo (Synthesis of Surface Active Agents and Their Applications),Maki Shoten, Tokyo (1964), A. M. Schwarts et al., "Surface ActiveAgents", Interscience Publications Corp., New York (1958), J. P. Sisleyet al., "Encyclopedia of Surface Active Agents", Vol. 2, ChemicalPublishing Co., New York (1964), "Kaimen Kassei Zai Binran" (Handbook ofSurface Active Agents), 6th Ed., Sangyo Tosho Co., Tokyo (Dec. 20,1966), etc.

These surface active agents can be used individually or in combinationwith each other. These surface active agents are generally used asantistatic agents, but in some cases, they are used for other purposes,for example, for improving the dispersibility, magnetic properties andlubricity, or as an auxiliary coating agent.

Formation of magnetic recording layer is carried out by dissolving theabove described composition in an organic solvent and then coating theresulting composition onto a support.

Suitable materials which can be used for this support are variousplastics, for example, polyesters such as polyethylene terephthalate,polyethylene-2,6-naphthalate and the like, polyolefins such aspolypropylene and the like, cellulose derivatives such as cellulosetriacetate, cellulose diacetate and the like, polycarbonates, etc., andnon-magnetic metals, for example, copper, aluminum, zinc, etc.

Such a non-magnetic support has a thickness of about 3 to 100 microns,preferably 5 to 50 microns in the form of a film or sheet and athickness of about 0.5 to 10 microns in the form of a disk or card. Inthe case of a drum form, its type is determined according to a recorderto be used.

The above described magnetic powder, binder, additives and solvent arewell blended or kneaded to prepare a magnetic coating composition. Forkneading, the magnetic powder and other components are charged in akneading machine simultaneously or separately. For example, a magneticpowder is added to a solvent containing a dispersing agent, kneaded fora predetermined period of time, then mixed with other components andkneaded sufficiently to prepare a magnetic coating composition.

Various kneading machines are used for the kneading and dispersing, forexample, two roll mills, three roll mills, ball mills, pebble mills,trommel mills, sand grinders, Szegvari attriters, high speed impellerdispersing machines, high speed stone mills, high speed mixershomogenizers, ultrasonic dispersing machines, etc. The kneading anddispersing techniques are described in T. C. Patton, "Paing Flow andPigment Dispersion", published by John Wiley & Sons (1964) and U.S. Pat.Nos. 2,581,414 and 2,855,156.

The foregoing magnetic recording layer can be coated on a support usingcoating methods such as air doctor coating, blade coating, air knifecoating, squeeze coating, dip coating, reverse roll coating, transferroll coating, gravure coating, kiss coating, cast coating, spray coatingand the like, and other coating methods can also be used. These methodsare described in "Coating Kogaku" (Coating Engineering), page 253 to277, published by Asakura Shoten, Tokyo (Mar. 20, 1971).

Typical organic solvents which can be used in the coating includesketones such as acetone, methyl ethyle ketone, methyl isobutyl ketone,cyclohexanone and the like; alcohols such as methanol, ethanol,propanol, butanol and the like; esters such as methyl acetate, ethylacetate, butyl acetate, glycol monoethyl ether acetate and the like;ethers and glycol ethers such as diethyl ether, glycol monoethyl ether,glycol dimethyl ether, dioxane and the like; aromatic hydrocarbons suchas benzene, toluene, xylene, and the like; chlorinated hydrocarbons suchas methylene chloride, ethylene choloride, carbon tetrachloride,chloroform, ethylene chlorohydrin, dichlorobenzene and the like; etc.

The present invention will be explained in detail with reference to thefollowing examples. It will be obvious to one skilled in the art thatvarious changes and modifications can be made in components, ratios,operational order and the like without departing from the spirit of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the following examples.

EXAMPLES

The following composition was in known manner dispersed, coated onto apolyethylene terephthalate base having a thickness of 20 microns,subjected to a surface finishing treatment and then slit to obtain amagnetic tape of 3/4 inch in width for U-matic VTR.

    ______________________________________                                        Co-doped FeO.sub.x (x = 1.4)                                                                       100    parts by weight                                   Vinyl Chloride-Vinyl Acetate-Vinyl                                                                 12                                                       Alcohol Copolymer                                                             Polyester type Polyurethane                                                                        12                                                       Polyisocyanate       8                                                        Carbon Black         5                                                        Chromium Oxide (Cr.sub.2 O.sub.3)                                                                  A                                                        Granularα-Fe.sub.2 O.sub.3                                                                   B                                                        Myristic Acid        C                                                        Amyl Stearate        D                                                        Methyl Ethyl Ketone  300                                                      ______________________________________                                    

As to samples of the present invention and comparative examples obtainedby varying the amounts A, B, C and D, using other dispersing agents andlubricants in place of the myristic acid and amyl stearate and usingother abrasives in place of the chromium oxide, the still lives and headabrasion at a low temperature (5° C.) and room temperature were measuredto obtain results shown in the following table, in which numeralsrepresented parts by weight.

    __________________________________________________________________________         Abrasive Cr.sub.2 O.sub.3 with                                                             Granular α-Fe.sub.2 O.sub.3 with                                                    Myristic Acid/Amyl                                   Mohs' hardness of                                                                          Mohs' hardness of                                                                         Stearate                                        Sample                                                                             about 8.5, A 5.5, B      C/D                                             __________________________________________________________________________     1   1.5          20          1/1                                              2*  1.5          20          1/0                                              3*  1.5          20          0/1                                              4*  1.5          20          0/0                                              5*  1.5          0           1/1                                              6*  1.5          0           0                                                7*  0            20          1/1                                              8*  0            20          0                                                9*  0            0           1/1                                             10*  0            0           0                                               11*  2            4           0/0.8                                           12*  6            0           0/0.8                                           13*  1.5          20          1*.sup.a /1*.sup.ii                             14*  1.5          20          1*.sup.a /1*.sup.iii                            15*  1.5          20          1*.sup.a /1*.sup.iv                             16*  1.5          20          1*.sup.b /1*.sup.i                              17*  1.5          20          1*.sup.b /1*.sup.ii                             18*  1.5          20          1*.sup.b /1*.sup.iii                            19*  1.5          20          1*.sup.b /1*.sup.iv                             20*  1.5          20          1*.sup.c /1*.sup.i                              21*  1.5          20          1*.sup.c /1*.sup.ii                             22*  1.5          20          1*.sup.c /1*.sup.iii                            23*  1.5          20          1*.sup.c /1*.sup.iv                             24*  1.5          0           1/1                                             25   1.5          5           1/1                                             26   1.5          10          1/1                                              1   1.5          20          1/1                                             27   1.5          25          1/1                                             28   1.5          30          1/1                                             29   1.5          50          1/1                                             Chroma output and audio sensitivity lower by about 3 dB.                      30*  0            21.5        1/1                                             31   MgO 1.5, 6** 20          1/1                                             32   1.5, 8-9**   20          1/1                                             33   α-Al.sub.2 O.sub.3 1.5, 9**                                                          20          1/1                                             34   SiC 1.5, 9** 20          1/1                                             __________________________________________________________________________    Measured at 5° C.                                                                     Measured at Room Temperature                                                                      Head Abra-                                 Initial Still Head Abrasion                                                                  Still Life after                                                                        Initial Still Life                                                                      sion at                                    Life           100 runnings in                                                                         at Room Temp.                                                                           Room Temp.                                 (min) (μm/100 hr)                                                                         VTR at 5° C. (min)                                                               (min)     (μm/100 hr)                             __________________________________________________________________________    120<  2        120<      120<      2                                          32    9.5      16        20        4                                          5     4        1>        15        3.5                                        3     10       1>        10        4                                          60    8        5         120<      3                                          2     8        1>        10        6                                          7     1                  15        2                                          1≧                                                                           1                  1>        2                                          2     Scraping of                                                                            Low Durability                                                                          2         Scraping of                                      Magnetic                     Magnetic                                   1≧                                                                           Layer              1>        Layer                                      25    9        20        45        5                                          30    23       25        55        15                                         1>    3.5      2         9         3                                          1>    4        1>        10        3.5                                        1>    3.5      1>        6         3                                          50    6        30        80        5                                          3     7        2         15        5                                          1>    6        1         7         4.5                                        1>    5        1         3         4                                          60    10       40        75        6                                          2     9        1         16        5                                          1>    7        1         11        5.5                                        1>    8        1         10        6                                          60    4        5         120<      3                                          120<  4        110       120<      3                                          120<  3        120<      120<      2                                          120<  2        120<      120<      2                                          120<  2        120<      120<      2                                          120<  3        120<      120<      2                                          120<  6        120<      120<      4                                          35    2        15        120<      1.5                                        120<  2        120<      120<      2                                          120<  2        120<      120<      2                                          120<  2        120<      120<      2                                          120<  3        120<      120<      2.5                                        __________________________________________________________________________     Note:                                                                         *Comparative Example                                                          **Mohs' hardness                                                              *.sup.a Myristic Acid as Dispersing Agent                                     *.sup.b Stearyl Alcohol as Dispersing Agent                                   *.sup.c Lecitin as Dispersing Agent                                           *.sup.i Amyl Stearate as Lubricant                                            *.sup.ii Fluorinated type Oil (KRYTOX 143Ycommercial name) as Lubricant       *.sup.iii Graphite as Lubricant                                               *.sup.iv Molybdenum Disulfide as Lubricant                               

The following matters will be apparent from the results of this table:

The magnetic tape of the present invention has a long still life as wellas an excellent durability under severer conditions, e.g. at a lowtemperature such as 5° C. in addition to room temperature. Furthermore,it shows a long still life measured even after running 100 times at 5°C. in a VTR and a suppressed head abrasion even at not only roomtemperature but also low temperatures.

Sample Nos. 11 and 12 are comparative examples of the prior art(Japanese Patent Application OPI No. 129935/1980) which are inferior tothe present invention in still life and head abrasion, in particular, inthe durability at low temperatures.

Sample Nos. 13 to 23 are comparative examples using other dispersingagents and lubricants in place of the fatty acid and fatty acid ester ofthe present invention, which are unfavourable in still life, headabrasion and properties at low temperatures.

In view of Sample Nos. 1 and 24 to 29, it will clearly be understoodthat the quantity of granular α-Fe₂ O₃ in preferably within a range of 5to 30% by weight to the magnetic powder to obtain balanced excellentproperties.

In view of Sample Nos. 30 to 34, it is apparent that even when usingother abrasive particles than Cr₂ O₃, there are given good properties ifthey have a Mohs' hardness of at least 6.

What is claimed is:
 1. A magnetic recording medium comprising a magneticrecording layer on the surface of a non-magnetic support, the magneticrecording layer comprising (i) a ferromagnetic powder, (ii) 1-10% byweight based on the weight of the ferromagnetic powder of an abrasivehaving a Mohs' hardness of at least 6 and a mean particle size of 0.05to 5 microns (iii) 5-30% by weight based on the weight of theferromagnetic powder of a non-magnetic powder which is granular α-Fe₂ O₃having a mean particle size of 0.2 to 1 micron and (iv) 0.1-2% by weightbased on the weight of the ferromagnetic powder of a saturated orunsaturated fatty acid having 6 to 22 carbon atoms and 0.1-2% by weightbased on the weight of the ferromagnetic powder of a fatty acid esterhaving a melting point of at most 60° C.
 2. The magnetic recordingmedium of claim 1, wherein the ferromagnetic powder is selected from thegroup consisting of fine powders of γ-Fe₂ O₃, Co-doped γ-Fe₂ O₃, Fe₃ O₄,Co-doped Fe₃ O₄, Berthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄, Co-dopedBerthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄, CrO₂, Co-Ni-P alloys,Co-Ni-Fe alloys, Co-Ni-Fe-B alloys, Fe-Ni-Zn alloys, Fe-Mn-Zn alloys,Fe-Co-Ni-P alloys and Ni-Co alloys.
 3. The magnetic recording medium ofclaim 1, wherein the abrasive having a Mohs' hardness of at least 6 isselected from the group consisting of fused alumina, silicon carbide,chromium oxide, corundum, synthetic corundum, diamond, syntheticdiamond, garnet and emery.
 4. The magnetic recording medium of claim 1,wherein the saturated or unsaturated fatty acid is selected from thegroup consisting of capric acid, lauric acid, myristic acid, oleic acid,stearic acid, erucic acid and behenic acid.
 5. The magnetic recordingmedium of claim 1, wherein the fatty acid having a melting point of atmost 60° C. is selected from the group consisting of methyl stearate,ethyl stearate, butyl stearate, amyl stearate, butyl palmitate, butylmyristate, oleyl oleate and butyl laurate.